1. Field of the Invention
The present invention relates to an optotype presenting apparatus for examining visual acuity and others of an examinee's eye (a patient's eye) by presenting optotypes or charts.
2. Description of Related Art
As an optotype presenting apparatus for presenting optotypes or charts for visual acuity test, various types of apparatus have been known such as a display type apparatus arranged to display test optotypes or charts on a liquid crystal display or the like (e.g. see JP2006-42978A) as well as an apparatus arranged to present optotypes or charts printed on paper or the like by illuminating them with fluorescent light and an apparatus arranged to project optotypes or charts on a screen. In the display type apparatus, test optotypes or charts appear in black on a white background of the display illuminated by backlight.
In the case where the background luminance (illuminance) of optotypes largely varies from apparatus to apparatus, such variations may influence visual acuity test results. High background luminance tends to induce a good visual acuity test result as compared with low background luminance. Accordingly, apparatus manufacturers previously adjust and fix the background luminance in a relatively narrow range allowing the luminance to appear to be almost the same. However, the manufacturers around the world offer various optotype presenting apparatuses with different background luminances of test optotypes. Under present circumstances, the range of luminance permissible in visual acuity testing apparatuses (hereinafter, a “standard range”) is considerably wide (80 to 320 cd/m2, at present). In an optician's shop or an ophthalmological clinic, when an optotype presenting apparatus offered by a different manufacturer from a manufacturer of an existing apparatus used heretofore is additionally adopted, the new apparatus may have very different background luminance from that of the existing apparatus. In this case, those apparatuses could not have compatibility with each other.
To avoid such a disadvantage, it is conceivable that the display type optotype presenting apparatus is additionally provided with a function for changing the background luminance of optotypes by adjusting backlight intensity (level) of the display in a similar way to a computer display. This luminance change is performed by changing voltage to be applied to the backlight.
It was however found that the backlight intensity of the display largely varied among apparatuses even by the same adjustment value of applied voltage. Accordingly, if an adjustment value for changing the backlight intensity is uniformly set among the apparatuses, the background luminance of some of the apparatuses may fall outside the standard range permissible for the visual acuity test, resulting in lowered reliability of the visual acuity test. In the case where a plurality of optotype presenting apparatuses is newly adopted in the same place (the same shop or clinic), it is hard to adjust the background luminances of those apparatuses to the same luminance level to provide compatibility.